The present invention relates to the art of consolidation/molding of polybenzazole (PBZ) polymers and molecular composite materials containing these polymers.
PBZ polymers, i.e., polybenzoxazole, polybenzothiazole and polybenzimidazole, and their synthesis are described in great detail in the following patents which are incorporated by reference: Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,703,103 (Oct. 27, 1987): Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,533,692 (Aug. 6, 1985); Wolfe et al., Liquid Crystalline Poly(2,6-Benzothiazole) Compositions, Process and Products, U.S. Pat. No. 4,533,724 (Aug. 6, 1985); Wolfe, Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,533,693 (Aug. 6, 1985): Evers, Thermoxidatively Stable Articulated p-Benzobisoxazole and p-Benzobisthiazole Polymers, U.S. Pat. No. 4,359,567 (Nov. 16, 1982): Tsai et al., Method for Making Heterocyclic Block Copolymer, U.S. Pat. No. 4,578,432 (Mar. 25, 1986) and 11 Ency. Poly. Sci. & Eng., Polybenzothiazoles and Polybenzoxazoles, 601 (J. Wiley & Sons 1988).
Shaped articles made from consolidated polybenzazole polymers and molecular composite materials containing the same are known to be useful in aircraft, electronic and other commercial applications where heat, chemical and radiation resistance are desired in conjunction with good mechanical and electrical properties.
Typically, materials which have a melting or softening point are suitable for consolidation. Those materials which show no transitions (melting or softening) below their decomposition temperatures, such as rigid and semi-rigid polybenzazoles, have been considered unsuitable for consolidation.
Many mixtures and variations of PBZ have been made attempting to improve its processing such as the following:
(a) block copolymers of PBZ with thermoplastic polymers, such as those described in U.S. Pat. No. 5,030,706, EPO Publication No. 0 388 803 (Sep. 26, 1990), copending U.S. application Ser. No. 547,650, filed Jul. 2, 1990 and U.S. application Ser. No. 562,781, filed Aug. 6, 1990:
(b) block copolymers of rigid PBZ with semi-rigid PBZ, such as those described in U.S. Pat. No. 4,578,432 (Tsai):
(c) molecular composites of PBZ with thermoplastic copolymers and non-rigid PBZ polymers, described in 11 Ency. Poly. Sci. at pages 631-632 "The Materials Science & Engineering of Rigid Rod Polymers" at pages 507-569: and
(d) semi-rigid PBZ polymers, described in 11 Ency. Poly. Sci. & Eng. at pages 601, 619 and 620.
Some of these mixtures and variations of PBZ are thermoplastic and some are not. But all are difficult to consolidate into a shaped article without creating substantial voids in the article.
Consolidation processes for molecular composite materials are disclosed in the literature. For example, Wang, et al. in the Journal of Polymer Material Science Engineering, Vol. 57, page 512-516, (1987), and in U.S. Pat. No. 5,021,517, discloses consolidating thin films or fibers of molecular composites of poly(p-phenylene polymers into larger specimens: and Ptak et al. in the Journal of Polymer Material Science Engineering, Vol. 57, page 517-521 ( 1987), disclose consolidating coagulated wet molecular composites into bulk form. However, the consolidation process disclosed by Wang, et al. could be used only for materials which contain a component having a melting point or glass temperature to produce a void-free material. In the process of Ptak et al., it takes a long time to consolidate the materials and the acid solvent used in the process is difficult to remove from the materials because of their low surface areas.
It would be highly desirable to provide a simple process which is suitable for making shaped PBZ article which are substantially void-free.